Flats the Flounder is a flounder who first appears as a cameo in "Sandy's Rocket," where he was one of SpongeBob's "alien hunting" victims. He also makes another minor appearance in "Texas," where he attended Sandy's Texas party. His only major appearance, however was in "The Bully," joins Mrs. Puff's boating school and wants to "kick SpongeBob's butt." SpongeBob takes this extremely seriously, reacting as if he were going to actually kill him. He makes many attempts to reason with and/or escape him, but to no avail. He even calls Patrick to protect him, only to find out that Flats is with him and that they were community college buddies. He also tries to go to Mrs. Puff to help, who talks to Flats (much to SpongeBob's dismay) and explains to SpongeBob that "kicking his butt" means that Flats wants to become his friend (this is obviously not true).
Flats goes so far as to chase SpongeBob out of school into town, even trying to run him over with a garbage truck. However, the garbage truck slips on a banana peel and flips over, critically injuring Flats. He wakes up in the hospital and learns that SpongeBob saved his life through CPR, yet he still wants to kick his butt. Eventually, Flatts corners SpongeBob in his own home, and proceeds to "kick his butt" by repeatedly hitting him. However, SpongeBob discovers that his spongy body simply absorbs the blows and does not harm him in anyway at all.
Flats continues to hit SpongeBob throughout the whole week until he finally passes out from exhaustion one morning at boating school. He is voiced by Thomas F. Wilson.